Moisture content of many particulate materials is an important parameter for processing or storing the particulate material. The water content of aggregate materials, particularly fine aggregates or sand, is an important parameter in the production of concrete because it is critical to concrete strength. Concrete sand is normally 3% to 10% water and the water content must be measured to determine how much sand and additional water to add to make good quality concrete. Uncontrolled variations in the moisture content of the aggregates is a major problem affecting the production of high specification concrete because relatively small changes in sand moisture have a significant effect upon the workability of the concrete in its plastic state and upon the compressive strength of the cured concrete.
The prior art discloses a variety of systems for gauging the moisture content of particulate materials. These systems include the use of infrared techniques, the detection of neutron backscatter and the measurement of electrical conductivity. Many devices utilize a parallel resonant, electronic circuit which includes a capacitor formed by spaced capacitor plates. A sample of the particulate material is interposed between the capacitor plates. Because the dielectric constant of the particulate is approximately proportional to its moisture content, capacitance changes can be used to detect moisture. To detect capacitance, an AC signal is applied to the resonant circuit, which includes the capacitor having the interposed particulate as a dielectric. The applied AC signal has a frequency near the resonant frequency of the resonant circuit. Because the impedance across a resonant circuit is reduced when the capacitance changes, voltage measurements across the tuned circuit are approximately inversely proportional to moisture content when the resonant circuit is connected in a voltage divider circuit.
In the earliest form of this concept practiced in the prior art, a sample of the material being tested was positioned between the capacitor plates. This concept is illustrated, for example, in U.S. Pat. No. 1,822,604, which illustrates a pair of parallel, planar, capacitor plates. This concept is extended to cylindrical embodiments in which the material being tested was placed within or flowed through an outer cylindrical capacitor plate, with the second capacitor plate formed as a smaller coaxial rod within the pipe. Such structures are illustrated by U.S. Pat. Nos. 2,720,624; 2,904,751; and 3,684,952. These systems provide acceptable performance for those materials for which flow through a pipe is practical.
However, for some materials, such as sand utilized in mixing concrete, flow through a pipe is impractical because of their flow characteristics, their abrasive nature and the large quantity of such materials which are used. The use of batch testing of small samples is also impractical for some applications because of the difficulty of obtaining and measuring representative samples and the inconvenience of repeatedly filling and emptying the testing device.
The prior art further developed the above capacitor testing system by forming the capacitor in a generally planar configuration so that,the two capacitor plates are located in the same plane, such as on the wall of a container, but are spaced from each other. As a consequence, an electric field having semi-circular and semi-elliptical electric field lines project from one side of the planar capacitor through the particulate material. Illustrative of that technique are U.S. Pat. Nos. 3,046,479; 3,252,086; and 4,560,923.
The use of the planar capacitor system avoids the need for the batch testing of samples or the need to flow the material through a pipe. That system can give acceptable results where the particulate material has a moisture content which is homogeneously distributed throughout its container, so that the moisture content measured at the container wall where the capacitor plates are positioned in their planar configuration is the same as the moisture content elsewhere.
However, the modern, ready-mix concrete industry uses automated, batch processing equipment. In the modern automated system, the sand is retained in a storage bin and is dispensed from the bin through a controlled feed gate at the bottom of the bin. Automatic control systems weigh the material as it is fed and use the weight information to control the feed gate. The control system also measures the moisture content of the sand and determines the quantity of other materials, including water, to add to the mix.
In an automated system, the sand flows principally through the central portion of the bin and feed gate at high flow rates on the order of 1,000-1,500 pounds per second. The sand located adjacent the walls of the bin may remain relatively static and thus does not have a moisture content which is necessarily representative of the moisture content of the sand actually being dispensed. This eliminates the opportunity to use the planar capacitor configurations of the prior art because they would detect the moisture in the static boundary layer of sand rather than in the sand being dispensed. Furthermore, an attempt to use parallel planar or coaxial capacitor plates between which flow of the sand could be effected is also impractical because of the likely obstruction of the sand flow, the damage and wear upon such capacitor plates, and the likelihood that the plates would cause localized regions .of static interrupted flow and voids, causing erroneous readings much like measurements made at the walls of the bin.
It is therefore an object and feature of the present invention to provide an improved moisture measurement gauge which will measure the moisture content of the particulate material in the region where the material actually being utilized is flowing and to do so in a manner which does not impede the flow or create immobilized accumulations of particulate material or voids in the vicinity of the particulate being measured which would cause inaccurate measurements.
It is another Object and feature of the present invention to provide a simple, low cost moisture measurement gauge having improved sensitivity and which eliminates the lossy electrical resistance, cost and mechanical instabilities associated with a lumped inductor.
Another object and feature of the present invention is to provide a moisture detection gauge which is rugged and durable and permits the delicate electronic circuit to be positioned remotely from the moisture sensing elements where the circuitry is both better protected and more easily serviced.
Another object and feature of the present invention is to provide a moisture sensing probe which inherently performs an impedance transformation so that the effective impedance of the tuned circuit is directly proportional to, rather than inversely proportional to, the sand moisture content in order to enhance the simplicity of the detection and display circuitry.